Cooling apparatus for an infra-red detector

ABSTRACT

The invention relates to infrared sensitive devices, such as infrared telescopes embodying a target, including photoelectrically sensitive material, for receiving infrared energy. The target is mounted on the outer surface of one of three spaced walls which form two cavities extending over the area of the target surface, the walls being transparent to infrared radiation so that such radiation can pass through them to the target. An inlet tube is provided having an orifice opening into an outer region of one of the cavities for emitting compressed gas into that cavity. The intermediate wall between the two cavities has a central aperture and the other cavity has an annular exhaust aperture, so that the gas emitted into the first cavity can pass through the aperture in the central area of the intermediate wall into the second cavity and disperses radially outward through the exhaust aperture, the ensuing expansion of the gas being effective to cool the target. Ducting is provided to lead the exhausted gas over the inlet tube to precool the gas before it is emitted from the orifice and also to guide the exhausted gas on to the target and the outer surface of a cavity wall to prevent misting thereof.

United States Patent 72] Inventors Victor Mich-2| Farmer Crowthorne; John Alirul Gurdler, Llghtwater; John M Knigli, Teddington; Robert Nlchobon Oswald, Hayes, all of, England [21 1 Appl. No. 674,055 [221 Filed Oct. 4, 1967 [45] Patented Aug. 31, 1971 [73] Assignee ElectrlcJrMdcallndnstrleslinited Middleaex, W [32] Priority 0. 12, 1966 [33] Great Britain [31 45605! [54] COOLING APPARATUS FOR AN FHA-RED DE'I'ECI'OR 11 Claim; 1 Drawing Fig.

[52] US. 250/833 ll [51] Int. Gtllt 1/16 [50] FleldotSear-ch ..25 0l83.3 IR

[56] References Cied UNITED STATES PATENTS 3,174,037 3/1965 Demorest et al 250/833 IR 3,258,602 6/1966 Promish 250/833 [R Primary Examiner-Rodney D. Bennett, Jr. Assistant Examiner-Joseph G. Baxter Attorney- Fleit, Gipple & Jacobson ABSTRACT: The invention relates to infrared sensitive devices, suchas infrared telescopes embodying a target, including photoelectrically sensitive material, for receiving infrared energy. The target is mounted on the outer surface of one of three spaced walls which form two cavities extending over the area of the target surface, the walls being transparent to infrared radiation so that such radiation can pass through them to the target. Au inlet tube is provided having an orifice opening into an outer region of one of the cavities for emitting compressed gas into that cavity. The intermediate wall between the two cavities has a central aperture and the other cavity has an annular exhaust aperture, so that the gas emitted into the first cavity can pass through the aperture in the central area of the intermediate wall into the second cavity and disperses radially outward through the exhaust aperture, the ensuing expansion of the gas being efiective to cool the target. Ducting is provided to lead the exhausted gas over the inlet tube to precool the gas before it is emitted from the orifice and also to guide the exhausted gas on to the target and the outer surface of a cavity wall to prevent misting thereof.

COOLING APPARATUS FOR AN INFRA-RED DETECTOR The present invention relates to infrared sensitive devices including for example a photoconductive surface.

It is well known that infrared sensitive photoemissive and photoconductive surfaces are preferably operated at a rela- It is moreover desirable to cool the target surface to reduce spurious thermal responses from the target. To reduce these spurious responses, it is important to cool the target surface uniformly so that the whole of the target surface is brought to approximately the same temperature, and the cooling apparatus should be such that there is no substantial obstruction or distortion of the infrared energy incident on the target.

An object of the present invention is to provide infrared sensitive devices with improved cooling apparatus, with a view to fulfilling one or more of the above requirements.

According to the present invention there is provided an infrared sensitive device embodying a target having a surface for receiving infrared material and cooling means for said target, said cooling means comprising: I

a. three spaced walls forming two cavities which extend over the area of said target surface,

b. an outer one of said walls being in conductive relationship with said surface,

c. a central area of the intermediate wall being apertured to allow gas to pass from a first of said cavities to the second of said cavities,

d. means for emitting compressed gas into an outer region of said first cavity,

e. means for exhausting gas from angularly spaced points in the outer region of said second cavity, so that gas emittedin to said first cavity passes into said second cavity in the central region thereof, and disperses radially outward to be exhausted from the outer region of said second cavity.

Preferably the gas exhausted through said exhaust means is caused to cool the compressed gas prior to its emission into the first of said cavities, and preferably the cavity into which the compressed gas is emitted is the cavity nearer said target surface and the emitting means is arranged to emit the gas with a substantial tangential component into said cavity to produce a gyration of the gas towards said orifice means.

In order that the present invention may be clearly understood and readily carried into effect it will now be described with reference to the accompanying drawing the single FIGURE of which shows in diagrammatic form one example of an infrared telescope including cooling apparatus according to one embodiment of the present invention.

Referring to the drawing the telescope shown comprises a tubular body 1 having at one end thereof an objective lens 2 and at the other end an eyepiece lens system 3. The objective lens 2, which is of a material which transmits infrared light, forms an infrared image on the photoconductive layer 4 of a target which is in the form of a solid state image converter. The image converter further includes an electroluminescent layer 5, the layers 4 and 5 being located between two transparent conductive electrodes 20 and 21 to which suitable biasing potentials can be applied. The leads for supplying the biasing voltages to the converter have been omitted from the drawing. In operation of the image converter, infrared radiacasing 7 having an annular part which fits closely round the helical coil of the tube 6 and surrounds the image converter, and a base portion comprising three spaced walls 22,8 and23. The walls form two cavities 24 and 25 separated by the wall 8 in which is formed a central orifice 9. The wall 22 is in contact with the conductive electrode 21 and is thence in close thermal conductive relationship with the photoconductive layer 4. The outer perimeter of the cavity 25, which is the cavity more remote from the photoconductive layer 4, has round. its periphery an annular exhaust aperture 27 which communicates with the annular part 26 of the casing 7. The other end of the annular part 26 of the casing 7 has an annular exhaust aperture 12 and located opposite this aperture 12 is a baffle 13 which it guides gas exhausted through the aperture 12 partly to the surface of the image converter provided with the conductive electrode 2l,"and partly past spacers 14 to the other surface of the wall 23. Further exhaust apertures 15 are provided on the tubular body 1 whereby gas can leave the interior of the telescope and enter a manifold 16 through which gas can be passed via a valve 17 to the atmosphere or returned for reuse. The cold region of the telescope, in which the cooling apparatus is provided, is surrounded by heat insulation 18 separated from the manifold 16 by spacers 19.

When it is desired to use the telescope, compressed contaminant free gas is passed into the tube 6 at the end 10 and is expanded from the orifice 11 in to the cavity 24 nearer to the photoconductive layer 22. The expansion cools the" gas rapidly. The end 11 of the tube 6 is oriented so that the gas is emitted substantially tangentially into the cavity 24, and as a consequence it gyrates or spirals in the cavity towards the orifice 9. At the center of the cavity 24 the gas passes through the central orifice 9 and then disperses radially outwards to the exhaust means 27 between the cavity 25 and the annular part 26 of the casing 7. The cold gas after being exhausted from the cavity 25 flows over the finned tube 6 precooling the compressed gas in that tube before it is emitted into the cavity 24. The turbulence of the cold gas in the cavity 24 ensures substantially uniform cooling of the photoconductive layer 4. The temperature of the gas falls until the rate of absorption of heat by the cooler is equal to the rate of the work which is done in expanding the compressed gas emitted from the tube 6. The rate of working depends on the pressure at which the gas is supplied to the tube 6, as both the rate of flow of gas through the orifice 11 and the amount of expansion of the gas will depend on the pressure. The passageof the cooled contaminantfree gas from the exhaust aperture 12 over the surface 21 of the image converter and over the outer surface of the wall 23 tends to prevent the formation of frost on these surfaces.

The .end 11 of the tube 6 is swaged down to reduce its diameter and restrict the flow of gas through the tube, so as to maintain the gas pressure within the tube and ensure that all of the cooling takes place when the gas expands on leaving th tube.

As the infrared energy forming the image on the image converter has to pass through the flat walls 22, 8 and 23 of the disclike cavities 24 and 25, it is necessary that these walls at least should be made of a material which is substantially transparent to infrared light. It is also desirable that these walls be relatively thin and of a material having a low refractive index so that distortion of the image due to the orifice 9 is reduced. One material which is suitable for making the casing 7 is Perspex. The tube 6 is made of cupronickel or stainless steel 'and is provided with a plurality of circular fins closely spaced along its length, which fins may be made of copper. Alte'rnatively, instead of being finned the tube 6 may be coiled longitudinally of itself within the annular part 26 of the case 7. Moreover instead of forming a helical coil of the tube 6, as in the annular part 26 of the casing 7, the tube may be otherwise formed as a meander.

Although the image converter shown consists of a layer 4 of photoconductive material adjacent to a layer 5 of electrolu-,

converter may be used instead. The single orifice 9 may be replaced by a number of orifices and the annular apertures forming the exhaust means 27, 12 and 15 may be replaced by a plurality of close but angularly spaced apertures. It is not necessary for the cooling apparatus to have substantial circular symmetry; it may be, for example, of square cross section.

It is possible to control the temperature of the image converter by adjustment of the pressure of the gas in the tube 6, and closed loop control of the temperature may be utilized by providing a temperature-sensitive element on the cooled surface of the image converter and using the temperature so measured in a feedback loop to control the pressure of the gas passed to the end 11 of the tube. The gas itself may be compressed contaminant free air at a pressure of, for example, 2,000 pounds per square inch, or compressed inert gas such as Argon at a similar pressure. The compressed gas may be stored in a suitable container to render the telescope portable. In an alternative arrangement the gas used may be recirculated by feeding back the gas from the manifold 16 to a suitable pump.

The power supply for the image converter may be derived from suitable batteries with a transistor high-voltage generator used if necessary to provide the operating voltages. In this way the telescope may be kept to a reasonably small size and weight so that it can be carried.

Although the invention has been described with reference to a specific embodiment in the form of an infrared telescope, it will be appreciated that it may be applied to other infrared optical devices such as binoculars, viewing screens, or the like. The invention is also applicable to infrared sensitive devices in which an electrical output is derived rather than visible light output.

We claim: 1. An infrared sensitive device embodying a target having a surface for receiving infrared energy and including photoelectrically sensitive material, and cooling means for said target, said cooling means comprising:

a. three spaced walls forming two cavities which extend over the area of said target surface, I

b. an outer one of said walls being in cdnductive relationship with said surface, 1

c. a central area of the intermediate wall being apertured to allow gas to pass from a first of said cavities to the second of said cavities,

d. means for emitting compressed gas into an outer region of said first cavity,

e. means for exhausting gas from angularly spaced points in the outer region of said second cavity, so that gas emitted into said first cavity passes into said second cavity in the central region thereof, and disperses radially outward to be exhausted from the outer region of said second cavity.

2. A device according to claim 1 in which said walls are transparent to infrared radiation so that such radiation can pass through said walls to said surface.

3. A device according to claim 1 in which said means for emitting compressed gas comprises an inlet tube formed with an orifice directed to emit compressed gas with a substantial tangential component into the outer region of said first cavity so that the gas gyrates to the apertured central area of said intermediate wall. I

4. A device according to claim 3 comprising a duct connected to said outer region of said second cavity to receive gas exhausted from said second cavity, said inlet tube being located in said duct so that exhaust gas cools said inlet tube.

5. A device according to claim 3, said inlet tube being formed as a helical coil surrounding said target.

6. A device according to claim 1 comprising means for guiding gas exhausted from said second cavity to said target, to reduce frosting thereon.

7. A device according to claim 1 comprising means for guiding gas exhausted from said second cavity to the outer surface of an outer one of said walls, to reduce frosting thereon.

. An infrared sensitive device embodying cooling apparatus according to claim 1 in which said target is incorporated in means for converting infrared energy into visible light energy.

9. An infrared sensitive device according to claim 1 comprising means for focusing an infrared image on said target through said walls.

10. An infrared sensitive device according to claim 8 comprising an electroluminescent layer for producing said visible light energy, and an eyepiece for viewing said electroluminescent layer.

11. An infrared sensitive device according to claim 8 in which said converting means comprises a photoconductive layer sensitive to infrared energy, an electroluminescent layer conductively coupled to said photoconductive layer, said layers being located between transparent conductive electrodes. 

1. An infrared sensitive device embodying a target having a surface for receiving infrared energy and including photoelectrically sensitive material, and cooling means for said target, said cooling means comprising: a. three spaced walls forming two cavities which extend over the area of said target surface, b. an outer one of said walls being in conductive relationship with said surface, c. a central area of the intermediate wall being apertured to allow gas to pass from a first of said cavities to the second of said cavities, d. means for emitting compressed gas into an outer region of said first cavity, e. means for exhausting gas from angularly spaced points in the outer region of said second cavity, so that gas emitted into said first cavity passes into said second cavity in the central region thereof, and disperses radially outward to be exhausted from the outer region of said second cavity.
 2. A device according to claim 1 in which said walls are transparent to infrared radiation so that such radiation can pass through said walls to said surface.
 3. A device according to claim 1 in which said means for emitting compressed gas comprises an inlet tube formed with an orifice directed to emit compressed gas with a substantial tangential component into the outer region of said first cavity so that the gas gyrates to the apertured central area of said intermediate wall.
 4. A device according to claim 3 comprising a duct connected to said outer region of said second cavity to receive gas exhausted from said second cavity, said inlet tube being located in said duct so that exhaust gas cools said inlet tube.
 5. A device according to claim 3, said inlet tube being formed as a helical coil surrounding said target.
 6. A device according to claim 1 comprising means for guiding gas exhausted from said second cavity to said target, to reduce frosting thereon.
 7. A device according to claim 1 comprising means for guiding gas exhausted from said second cavity to the outer surface of an outer one of said walls, to reduce frosting thereon.
 8. An infrared sensitive device embodying cooling apparatus according to claim 1 in which said target is incorporated in means for converting infrared energy into visible light energy.
 9. An infrared sensitive device according to claim 1 comprising means for focusing an infrared image on said target through said walls.
 10. An infrared sensitive device according to claim 8 comprising an electroluminescent layer for producing said visible light energy, and an eyepiece for viewing said electroluminescent layer.
 11. An infrared sensitive device according to claim 8 in which said converting means comprises a photoconductive layer sensitive to infrared energy, an electroluminescent layer conductively coupled to said photoconductive layer, said layers being located between transparent conductive electrodes. 